Studies of the retina in 6- and 22-month-old English setters with progressive blindness, ataxia, and muscle weakness demonstrated a marked accumulation of abnormal cytosomes within neurons and retinal pigmented epithelial cells. Ganglion cells contained abundant cytosomes with evenly spaced stacks of membranes; bipolar and amacrine cell cytosomes consisted of dense, amorphous material with closely spaced configurations of light and dark lines; cytosomes within photoreceptor cells contained faintly staining curved profiles. All three cytosomes resembled those previously reported in brain neurons of CCL dogs. In retinal pigmented epithelial cells, there were prominent accumulations of lamellar fragments, either free in the cytoplasm or incorporated into melanin granules. These retinal abnormalities are likely to be related to deficiences of peroxidase and defects of lipid peroxidation. The pathologic and biochemical changes seen in these dogs are similar in many respects to those reported in human patients with Batten disease. As such, these dogs provide a convenient model for the study disease mechanisms and for therapeutic approaches to blindness in Batten disease.